A light guide plate is one of the optical members to be used in a backlight unit for any one of various display apparatuses. A polycarbonate resin (which may hereinafter be abbreviated as “PC”) produced from bisphenol A has been used as a material for a light guide plate of a size as small as 1 to 7 inches. A PC produced from, for example, bisphenol A is suitable for a light guide plate to be mounted on, for example, a portable phone, a portable game machine, or a car navigation system because the PC is excellent in transparency and thermal resistance, has low hygroscopic property, and is excellent in mechanical characteristics and light conducting performance (see, for example, Patent Document 1).
In addition, an optical polycarbonate excellent in resistance to solder reflow has been known, and a light guide plate has been proposed as an application of the polycarbonate (see, for example, Patent Document 2).
In addition, a light guide plate is one of the optical members to be used in an illumination unit such as a backlight unit for any one of various display apparatuses, and an aromatic polycarbonate resin composition for a light guide plate obtained by blending an aromatic polycarbonate resin with a specific amount of a polyorganosiloxane having the following characteristics has been known (see, for example, Patent Document 3): the polyorganosiloxane has a phenyl group at any one of its side chains, has a branched siloxane structure, and has a specific kinetic viscosity at 25° C.
With the advent of reductions in thickness and size of a light source for use in an edge light type backlight, space savings based on a reduction in thickness have been demanded also of a light guide plate itself in recent years. However, when a polycarbonate resin composition described in Patent Document 1 or 2 is molded into a light guide plate by injection molding, it becomes difficult to achieve the target reduction in thickness of the light guide plate because the composition has low flowability.
In addition, in the case where a polycarbonate resin composition described in Patent Document 3 is used in a light guide plate, the light guide plate to be obtained has a good hue when the composition is molded at low temperature. However, when the temperature at which the composition is molded is increased in order that melt flowability sufficient for a reduction in thickness of a light guide plate may be secured, it becomes difficult to obtain a light guide plate having satisfactory performance owing to, for example, the following problem: the color tone of the light guide plate to be obtained on, in particular, a side opposite to the side on which light is incident becomes yellowish owing to the thermal deterioration of the carbonate resin of the composition, or a gas is generated at the time of the molding at high temperature. Accordingly, a molding resin material having the following characteristics has been requested: the material can be additionally reduced in thickness, and can be expanded into an additionally large size, as compared to a conventional material, and is excellent in light conducting performance.
By the way, a copolymer composed of α,α′-bis(4-hydroxyphenyl)-m-diisopropylbenzene and 2,2-bis(4-hydroxyphenyl)propane (bisphenol A) has been known (see, for example, Patent Document 4). Known applications of the copolymer are an optical lens (see, for example, Patent Documents 5 and 6) and an optical disk substrate (see, for example, Patent Document 7) because the copolymer has low water absorption property and low birefringence. At present, however, no material showing excellent light conducting performance and excellent moldability has been technically established yet.
Patent Document 1: JP 11-158364 A
Patent Document 2: JP 2005-60628 A
Patent Document 3: JP 2004-250557 A
Patent Document 4: JP 63-89539 A
Patent Document 5: JP 2003-192780 A
Patent Document 6: JP 2003-96179 A
Patent Document 7: JP 2002-348367 A